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Caption

Human polychromatic erythroblast (normoblast), coloured transmission electron micrograph (TEM). The stage in erythrocyte (red blood cell) development where the erythroblast possesses a nucleus and contains haemoglobin. The cell will lose the nucleus and become a reticulocyte (a large, immature red blood cell with retention of portions of the nucleus). In the process of red blood cell formation, a cell undergoes a series of 7 differentiations in the bone marrow. In all of these stages the developing red blood cells have a nucleus. Upon reaching the 7th stage, called a reticulocyte, the red blood cells have extruded their nucleus. The reticulocytes are released in to the blood stream where they become mature red blood cells. Red blood cells of most all other vertebrates have a nucleus. The erythroblast is the immediate precursor of a normal erythrocyte. Magnification: x1,635 when shortest axis printed at 25 millimetres.